1. Technical Field
The present disclosure relates to an assembled board with a high-density wiring conductor.
2. Background
A plan view and a main part enlarged cross-sectional view of the conventional assembled board B are shown in FIGS. 2A and 2B. Such an assembled board B is described, for example, in Japanese Unexamined Patent Application Publication No. 2013-172073. The assembled board B includes an insulating board 10, a conductor layer 12, and a solder resist layer 13. In a top view, the insulating board 10 includes a plurality of product regions X to be the individual wiring boards, and a marginal region Y surrounding the product regions X. Although twelve product regions X are illustrated in the assembled board B, the assembled board B actually includes several tens to several hundreds of product regions X.
In the insulating board 10, the insulating layers for the build-up 10b are laminated on the upper and lower surfaces of the insulating layer for the core 10a. A plurality of through holes 14 are formed in the insulating layer for the core 10a corresponding to the product region X. A through-hole conductor 12a including a part of the conductor layer 12 is filled inside the through hole 14. A plurality of via holes 15 are formed in the insulating layers for the build-up 10b corresponding to the product region X. A via conductor 12b including a part of the conductor layer 12 is filled inside the via hole 15.
The conductor layer 12 is formed on the surface of and inside the insulating board 10. The conductor layer 12 is formed, for example, by using a well-known plating method. Part of the conductor layer 12 formed on the upper surface of the insulating board 10 functions as the semiconductor-element connection pad 16. The electrode of the semiconductor element is connected to the semiconductor-element connection pad 16 through the solder. Part of the conductor layer 12 formed on the lower surface of the insulating board 10 functions as the external connection pad 17. The wiring conductor of the external electric circuit board is connected to the external connection pad 17 through the solder.
This causes the semiconductor element and the external electric circuit board to connect each other electrically, and causes the semiconductor element to be activated by transmission and reception of electrical signals through the conductor layer 12. The insulating board 10 where the conductor layer 12 is formed has a coefficient of thermal expansion of about 10 to 20 ppm/° C.
The solder resist layer 13 is formed on the upper and lower surfaces of the insulating board 10. The solder resist layer 13 formed on the upper surface includes the opening 13a exposing the central portion of the semiconductor element connection pad 16. The solder resist layer 13 formed on the lower surface includes the opening 13b exposing the central portion of the external connection pad 17. The resin paste or film, for example, formed of electrically insulating materials is applied or adhered onto the surface of the insulating layer for the build-up 10b to be thermally cured, whereby the solder resist layer 13 is formed. The solder resist layer 13 has a thickness of about 15 to 20 μm. The solder resist layer 13 has a coefficient of thermal expansion of about 60 ppm/° C.
By the way, as described above, when the solder resist layer 13 is heated to be cured, a stress due to the contraction of the solder resist layer 13 occurs on the upper surface side and the lower surface side of the insulating board 10 due to the difference in coefficients of thermal expansion between the insulating board 10 and the solder resist layer 13. In such a state, for example, when the volume of the conductor layer 12 on the upper surface side of the wiring board 10 is smaller than the volume of the conductor layer 12 on the lower surface side of the wiring board 10, the stiffness on the upper surface side of a smaller volume of the conductor layer 12 becomes small as compared to the stiffness on the lower surface side. Therefore, the assembled board B sometimes warps to the upper surface side due to the contraction stress of the solder resist layer 13.
In particular, in the corner portion of the assembled board B in a position away from the central portion of the assembled board B, the stress is large, and there is a tendency for the upward warpage to increase. For this reason, when a semiconductor element is mounted on the assembled board B, the height of the semiconductor-element connection pad becomes uneven. As a result, there is a problem that the electrode of the semiconductor element and the semiconductor-element connection pad cannot be fully joined through the solder, and that the semiconductor element cannot be stably operated.